Gianfranco Vidali

Potentials of physical adsorption

Abstract A summary of key data concerning physical adsorption potentials is presented. Over 250 gas-surface systems have been considered. For each, we tabulate (when available) the van der Waals adsorption coefficient (C3), the equilibrium distance (zm), well depth (D), and binding energy (Eb) of the laterally averaged interaction potential as deduced from analyses of experimental data and calculations. “Best values” are proposed for many systems. Our tables are preceded by an introduction to the experimental and theoretical methods used in deducing physical adsorption potentials.

Molecular hydrogen formation on astrophysically relevant surfaces

Recent experimental results about the formation of molecular hydrogen on astrophysically relevant surfaces under conditions close to those encountered in the interstellar medium are analyzed using rate equations. The parameters of the rate equation model are fitted to temperature-programmed desorption curves obtained in the laboratory. These parameters are the activation energy barriers for atomic hydrogen diffusion and desorption, the barrier for molecular hydrogen desorption, and the probability of spontaneous desorption of a hydrogen molecule upon recombination. The model is a generalization of the Polanyi-Wigner equation and provides a description of both first- and second-order kinetic processes within a single model. Using the values of the parameters that best fit the experimental results, the efficiency of hydrogen recombination on olivine and amorphous carbon surfaces is obtained for a range of hydrogen flux and surface temperature pertinent to a wide range of interstellar conditions.

Master Equation for Hydrogen Recombination on Grain Surfaces

Recent experimental results on the formation of molecular hydrogen on astrophysically relevant surfaces under conditions similar to those encountered in the interstellar medium provided useful quantitative information about these processes. Rate equation analysis of experiments on olivine and amorphous carbon surfaces provided the activation energy barriers for the diffusion and desorption processes relevant to hydrogen recombination on these surfaces. However, the suitability of rate equations for the simulation of hydrogen recombination on interstellar grains, where there might be very few atoms on a grain at any given time, has been questioned. To resolve this problem, we introduce a master equation that takes into account both the discrete nature of the H atoms and the fluctuations in the number of atoms on a grain. The hydrogen recombination rate on microscopic grains, as a function of grain size and temperature, is then calculated using the master equation. The results are compared to those obtained from the rate equations, and the conditions under which the master equation is required are identified.

Efficiency of Molecular Hydrogen Formation on Silicates

We report on laboratory measurements of molecular hydrogen formation and recombination on an olivine slab as a function of surface temperature under conditions relevant to those encountered in the interstellar medium. On the basis of our experimental evidence, we recognize that there are two main regimes of H coverage that are of astrophysical importance; for each of them we provide an expression giving the production rate of molecular hydrogen in interstellar clouds.

Laboratory Synthesis of Molecular Hydrogen on Surfaces of Astrophysical Interest

We report on the first results of experiments to measure the recombination rate of hydrogen on surfaces of astrophysical interest. Our measurements give lower values for the recombination efficiency (sticking probability S times the probability of recombination upon H-H encounter, ?) than model-based estimates. We propose that our results can be reconciled with average estimates of the recombination rate [(1/2)nHngvHAS?] from astronomical observations, if the actual surface of an average grain is rougher, and its area bigger, than the one considered in models.

Formation of Carbon Dioxide by Surface Reactions on Ices in the Interstellar Medium

The formation of carbon dioxide by surface reactions has been investigated experimentally in conditions close to those encountered in the interstellar medium. Carbon monoxide and oxygen atoms have been concurrently deposited on a copper substrate at 5 K. The formation and release in the gas phase of carbon dioxide have been monitored by a mass spectrometer during a programmed desorption. Our measured rates and energy barrier show that it is possible to make CO2 efficiently in ice mantles on grains without the intervention of energizing (UV or energetic particles) agents.

Laboratory measurements of molecular hydrogen formation on amorphous water ice

We report on an experimental study of the formation of hydrogen molecules by surface recombination of adsorbed H atoms on amorphous water ice under conditions closely simulating those encountered in astrophysical environments. Our results show that hydrogen recombination via surface reactions on icy mantles on grains is able to account for H2 formation in dense cloud environments.

Molecular Hydrogen Formation on Ice Under Interstellar Conditions

The results of experiments on the formation of molecular hydrogen on low-density and high-density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low-density ice gives rise to a broader spectrum of energy barriers compared to the high-density ice. Inserting these parameters into the rate equation model under steady-state conditions, we evaluate the production rate of molecular hydrogen on ice-coated interstellar dust grains.

The interaction between an atom and a surface at large separation

Abstract A simple expression is generated for the coefficient C 3 of the asymptotic dispersion interaction ~ z −3 between an atom and a surface. The basis of the derivation is the assumption of simple forms for the frequency dependence of the polarizability of the atom and the dielectric function of the solid. The expression yields the known value of C 3 within a few percent for all cases for which it has been calculated previously. We use it to generate C 3 values for a large number of systems not previously treated.

Formation of Molecular Hydrogen on Amorphous Water Ice: Influence of Morphology and Ultraviolet Exposure

In this paper, we report on the formation of molecular hydrogen on different types of amorphous water ice. We show that mass spectra of desorbing molecules upon formation are sensitive to the way in which ice is deposited on a cold substrate, to its thermal history, and to the action of UV photons. Implications that these results bear on H2 formation in dense quiescent clouds are presented and discussed. Subject headings: astrochemistry — dust, extinction — ISM: molecules — methods: laboratory — molecular processes The formation of the hydrogen molecule, the most abundant in the universe, is one of the fundamental processes occurring in the interstellar medium. It has been recognized that it cannot form efficiently in the gas phase, because upon formation the release of the energy excess via radiative decay is not allowed by selection rules, and that the role of dust grains as catalysts is crucial to explain its abundance.